https://motherboard.vice.com/en_us/article/yw984x/whats-that-weird-cloud-thats-been-looming-over-mars-for-weeks

For over a month, Mars orbiters have been watching a weird Martian cloud that resembles a booger and is so large that it can be seen with telescopes on Earth.

Suspended over the western slope of Arsia Mons, an enormous volcano near the red planet’s equator, the elongated cloud stretches for about 930 miles, and was first spotted by the European Space Agency’s (ESA) Mars Express orbiter on September 13. Though the cloud looks like the kind of volcanic plumes huffed out by Earth’s active volcanoes, Arsia Mons is long extinct—its last eruption is estimated to have occurred around 50 million years ago.